Rick L. Morrison

Five-stage free-space optical switching network with field-effect transistor self-electro-optic-effect-device smart-pixel arrays

The design, construction, and operational testing of a five-stage, fully interconnected 32 × 16 switching fabric by the use of smart-pixel (2, 1, 1) switching nodes are described. The arrays of switching nodes use monolithically integrated GaAs field-effect transistors, multiple-quantum-well p-i-n detectors, and self-electro-optic-device modulators. Each switching node incorporates 25 field-effect transistors and 17 p-i-n diodes to realize two differential optical receivers, the 2 × 1 node switching logic, a single-bit node control memory, and one differential optical transmitter. The five stages of node arrays are interconnected to form a two-dimensional banyan network by the use of Fourier-plane computer-generated holograms. System input and output are made by two-dimensional fiber-bundle matrices, and the system optical hardware design incorporates frequency-stabilized lasers, pupil-division beam combination, and a hybrid micro-macro lens for fiber-bundle imaging. Optomechanical packaging of the system ut lizes modular kinematic component positioning and active thermal control to enable simple rapid assembly. Two preliminary operational experiments are completed. In the first experiment, five stages are operated at 50 Mbits/s with 15 active inputs and outputs. The second experiment attempts to operate two stages of second-generation node arrays at 155 Mbits/s, with eight of the 15 active nodes functioning correctly along the straight switch-routing paths.

Symmetries that simplify the design of spot array phase gratings

The design of advanced diffraction gratings that produce spot arrays is highly influenced by the computational capabilities that are available to the designer. This is due to the increased pattern complexity that is required for larger spot arrays or higher efficiencies. Symmetries that lead to a significant reduction in the design complexity can be incorporated into the grating pattern. In addition, a translational symmetry leads to the highly desired result of even-numbered spot arrays. We examine the symmetries that can be applied to both general-and discrete-level design parameterization.

Six-stage digital free-space optical switching network using symmetric self-electro-optic-effect devices

We describe the design and demonstration of an extended generalized shuffle interconnection network, centrally controlled by a personal computer. A banyan interconnection pattern is implemented by use of computer-generated Fourier holograms and custom metallization at each 32 × 32 switching node array. Each array of electrically controlled tristate symmetric self-electro-optic-effect devices has 10,240 optical pinouts and 32 electrical pinouts, and the six-stage system occupies a 9 in. × 12.5 in. (22.9 cm × 31.7 cm) area. Details of the architecture, optical and mechanical design, and system alignment and tolerancing are presented.

Time-domain images.

We demonstrate that, using a hologram, we can convert a spatial x-y image into an x-t image, where one spatial dimension is now transformed into the time domain. In particular, we generate the temporal equivalent of our corporate logo, where every pixel of the image is now represented by a short optical pulse.

Experimental investigation of a free-space optical switching network by using symmetric self-electro-optic-effect devices.

A prototype digital free-space photonic switching fabric is demonstrated. It consists of three cascaded 16 x 8 arrays of symmetric self-electro-optic-effect devices that are used as logic gates that implement part of a multistage interconnection network. We discuss architecture, device tolerancing, optical system design, and optomechanical design. This optical circuit is successfully configured as a fully operational array of 32 independent 2 x 2 nodes and operates at 100 kHz.

Ultrathin cameras using annular folded optics

We present a reflective multiple-fold approach to visible imaging for high-resolution, large aperture cameras of significantly reduced thickness. This approach allows for reduced bulk and weight compared with large high-quality camera systems and improved resolution and light collection compared with miniature conventional cameras. An analysis of the properties of multiple-fold imagers is presented along with the design, fabrication, and testing of an eightfold prototype camera. This demonstration camera has a 35 mm effective focal length, 0.7 NA, and 27 mm effective aperture folded into a 5 mm total thickness.

Beam array generation and holographic interconnections in a free-space optical switching network

Free-space photonic switching systems that optically interconnect large arrays of simple processing elements have already been demonstrated [IEEE Photon. Technol. Lett. 2, 438,600 (1990); Appl. Opt. 31, 5431 (1992); Electron. Lett. 27, 1869 (1991)]. In these system experiments, diffractive optical elements served as critical components that provided functionality not easily assumed by conventional optics. In the latest optical switching network, binary phase gratings were used to generate arrays of uniformintensity beams to illuminate modulators in the processor array. In addition, space-invariant binary phase grating designs were integral in forming the Banyan interconnection network used to link arrays in the system. Here we discuss the function, design, and performance of these diffractive elements.

Optomechanics of a free-space photonic switching fabric: the system

Parts of a multistage switching network were implemented by optically interconnecting arrays of symmetric self electro-optic effect devices. In an experiment completed last Spring, three 16 X 8 arrays of S-SEEDs, all operating as logic gates, were optically connected. A fully-interconnected switching fabric using six 32 X 32 S-SEED arrays is currently being tested. These are the latest in a series of experiments to investigate and develop this technology, and they substantially involve optomechanics. The practical realization of this technology represents a challenge to modern optomechanics due to the required precision, stability, and number of components involved. An overview of free-space photonic switching and the required experimental hardware subsystems is presented, followed by details of the optical systems to interconnect the switching device arrays and the mechanical systems which locate and position the optics and devices. The tolerancing analysis used in these systems is reviewed and comparisons between the two systems are made.

Shuffle-equivalent interconnection topologies based on computer-generated binary-phase gratings

Several different shuffle-equivalent interconnection topologies that can be used within the optical link stages of photonic-switching networks are studied. These schemes include the two shuffle, the two banyan, and the segmented two shuffle, which can be used to interconnect two-input, two-output switching nodes. The schemes also include the four shuffle and the four banyan, which can be used to interconnect four-input, four-output switching nodes. (Note: The segmented two shuffle and the four banyan are novel interconnection topologies that were developed to satisfy some of the constraints of free-space digital optics). It is shown that each of these interconnection topologies can be implemented by the use of relatively simple imaging optics that contain space-invariant computer-generated binaryphase gratings. The effects of node type and interconnection topology on the laser power requirements and the optical component complexity within the resulting systems are also studied. The general class of networks nown as extended generalized shuffle networks is used as a baseline for the analysis. It is shown that (2, 1, 1) nodes and (2, 2, 2) nodes connected by two-banyan interconnections can produce power-efficient and cost-effective systems. The results should help identify the architectural trade-offs that exist when a node type and an interconnection topology are selected for implementation within a switching system based on free-space digital optics.

Pupil plane multiplexing for multi-domain imaging sensors

We describe an approach to polarimetric imaging based on a unique folded imaging system with an annular aperture. The novelty of this approach lies in the systems collection architecture, which segments the pupil plane to measure the individual polarimetric components contributing to the Stokes vectors. Conventional approaches rely on time sequential measurements (time-multiplexed) using a conventional imaging architecture with a reconfigurable polarization filter, or measurements that segment the focal plane array (spatial multiplexing) by super-imposing an array of polarizers. Our approach achieves spatial multiplexing within the aperture in a compact, lightweight design. The aperture can be configured for sequential collection of the four polarization components required for Stokes vector calculation or in any linear combination of those components on a common focal plane array. Errors in calculating the degree of polarization caused by the manner in which the aperture is partitioned are analyzed, and approaches for reducing that error are investigated. It is shown that reconstructing individual polarization filtered images prior to calculating the Stokes parameters can reduce the error significantly.